Industrial radiography is a technique for the non-destructive testing and analysis of defects in components such as glass, paper, wood or metal components. This technique is widely used in aeronautics, the nuclear industry or the petroleum industry as it makes it possible to detect welding defects or defects in the texture of materials in aircraft, nuclear reactor or pipeline components.
This technique consists of exposing a metallic component to be analyzed to an ionizing radiation, in general X or .gamma. rays, with an energy lying between 10 and 15,000 kV. With this technique it is therefore necessary to use special radiographic products which are sensitive to this ionizing radiation.
The sensitivity of radiographic emulsions to X or .gamma. rays is due to the absorption of some of these rays by the silver halide grains, which causes a secondary emission of electrons which will form an internal latent image. Consequently the ionizing radiation is effective only to the extent it can be absorbed by these grains.
Unfortunately, it is known that the major part of the ionizing radiation passes through the silver halide grains without being absorbed. Only a very small part of the incident radiation (less than 1%) is absorbed and participates in the formation of developable latent image seeds.
It is known to employ an intensifier with an industrial radiographic product. The intensifier typically takes the form of a metallic foil that intercepts a portion of the X or .gamma. radiation and emits electrons that interact with the silver halide grains to form latent image sites.
These intensifiers are not to be confused with intensifying screens used in medical diagnostic radiographic products. In medical diagnostic imaging much lower energy levels of X radiation are employed. Again, only a small portion of the X radiation is absorbed by the silver halide grains. To increase absorption an intensifying screen is employed that absorbs X radiation and emits light. This is achieved by coating phosphor particles in a binder and coating on a support to form the intensifying screen. When light is emitted to the intensifying screen to the medical diagnostic radiographic product that lies outside the spectral region of native sensitivity of the silver halide grains, then a spectral sensitizing dye is employed having its maximum absorption wavelength matched to a primary emission wavelength of the intensifying screen. To transfer energy from the dye, where emitted light is absorbed, to the silver halide grains, it is necessary to adsorb the dye to the surface of the silver halide grains.
In medical diagnostic imaging the high surface to volume ratios of tabular grains allows higher amounts of sensitizing dye to be absorbed, and tabular grain emulsions are therefore generally preferred. Products for industrial radiography instead rely upon the silver halide itself to capture latent image forming radiation. Therefore, nontabular grains (regular grains and irregular grains with low aspect ratios) are most commonly employed. However, films for industrial radiography comprising tabular grain emulsions are known. U.S. Pat. No. 4,883,748 describes a film for industrial radiography in which the silver halide emulsion comprises silver halide grains having an average aspect ratio less than or equal to 5 (and preferably between 1 and 3) and whose surface region contains more iodide than the internal region.